1. Field of the Invention
The present invention relates to a nonreciprocal circuit device for use in the microwave band such as, for instance, an isolator or a circulator.
2. Description of the Related Art
Generally, a lumped constant isolator, used in mobile communication equipment such as mobile telephones, has a function which allows signals to pass only in the transmission direction while preventing transmission in the reverse direction. Furthermore, given the recent usage of mobile communication equipment, there are growing demands for smaller, lighter and less expensive devices. In the case of the isolator, there are similar demands for a smaller, lighter and cheaper device.
Conventionally, as shown in FIG. 6, this type of lumped constant isolator has a structure comprising top and bottom yokes 50 and 51 which contain, in sequence from the top, a permanent magnet 52, a central electrode body 53, a matching circuit board 54 and a ground board 55. The central electrode body 53 comprises three central electrodes 57.. which intersect in an electrically insulated state on a disc-shaped ferrite 56.
Furthermore, the matching circuit board 54 comprises a rectangular thin-board dielectric substrate 54a, having a round hole 54b, which the central electrode body 53 is inserted into, formed in the center thereof; and capacitor electrodes 58.., which input/output ports P1-P3 of the central electrodes 57 are connected to, formed around the round hole 54b in the dielectric substrate 54a. Further, an end resistance film 59 is connected to the port P3.
However, since the above conventional matching circuit board 54 requires forming the round holes 54b in the thin-board dielectric substrate 54a and patterning the central electrodes 57, there is a problem of complex processing during manufacture and assembly, increasing costs.
A further problem is that the parts other than the capacitor electrodes 58 unnecessarily increase the area and weight of the conventional dielectric substrate 54a, making it more difficult to produce a smaller and lighter device. In this connection, recently there is a demand for reducing the weight of isolators to the milligram level.
Yet another problem of the conventional matching circuit board 54 is that, since the capacitor electrodes 58 are formed on a dielectric substrate 54a having high permittivity, adjacent capacitor electrodes 58 are prone to electrostatic coupling Cp, which is damaging to the attenuation properties of the isolator outside the band.
In another conventional isolator, a single plate capacitor, comprising opposing electrodes provided on either side of a dielectric substrate so as to completely cover the surfaces thereof, is used as each of the the capacitors in lieu of the matching circuit board.
This single plate capacitor can be manufactured by forming electrodes on the two main surfaces of a motherboard, which comprises a large flat board, and cutting the motherboard to predetermined dimensions. Such a single plate capacitor can therefore be mass-produced. Consequently, processing and handling are easier than when round holes and multiple capacitors are provided to a conventional dielectric substrate, and cost can be reduced. In addition, since electrodes are formed over the entire faces of the substrate, unnecessary increase of area and weight can be eliminated, thereby enabling the isolator to be made smaller and lighter by a proportionate amount. Moreover, since the capacitors are provided separately, it is possible to prevent electrostatic coupling between them and thereby avoid deterioration of attenuation properties outside the band.
FIGS. 4 and 5 show an example of an isolator using a single plate capacitor and are not the prior art. Like members corresponding to those in FIG. 6 are designated by like reference characters. This isolator comprises a resin terminal block 60, having a round hole 61 provided in the base wall 60a thereof, the central electrode body 53 being inserted into the round hole 61; rectangular single plate capacitors C1-C3, provided on the periphery of the round hole 61 so as to surround the central electrode body 53; and a single plate resistor R.
As shown in FIG. 5, when the single plate capacitors C1.about.C3 are provided around the central electrode body 53, unwanted vacant spaces 62 are created therebetween. This is an obstacle to making the device smaller and lighter, and the demand mentioned above cannot be fulfilled.
Moreover, although the above single plate capacitors C1-C3 enable the isolator to be made smaller and lighter than the conventional device, a considerable amount of space is nevertheless taken up with respect to the whole of the isolator since the electrode area is determined by the required matching capacitance. This is a further obstacle to making the device small and light.
In order to reduce the size of the capacitors themselves, countermeasures such as the following have been considered and implemented: (1) use a high-permittivity material as the dielectric substrate; (2) further reduce the thickness of the dielectric substrate; (3) use laminated-chip capacitors.
However, in the case of (1), material having maximum permittivity of 100-120 is already being used. Material of even higher permittivity has unsuitable temperature characteristics and high-frequency characteristics decline, thus loss at the microwave band becomes considerably large. For these reasons, such material cannot be employed.
Furthermore, in the case of (2), a substrate of approximate thickness 0.2 mm is generally used. Reducing the thickness even further would cause an extreme reduction in the strength of the substrate, worsening yield and consequently lowering productivity as well as lowering the reliability of product quality.
Finally, in the case of (3), laminated capacitors generally have Q of 20-100 at the microwave band. This is much lower than the single plate capacitor using dielectric material for high-frequency, which has Q of more than 200, causing further loss of the characteristics of isolator. Furthermore, although the conventional laminated capacitor has relatively small top area S of approximately 0.5 mm.sup.2, it is approximately 0.5 mm tall, and hence has volume V of 0.25 mm.sup.3. By contrast, the single plate capacitor has S of 1.2 mm.sup.2 and V of approximately 0.24 mm.sup.3. Therefore, the size reduction achieved when using a laminated capacitor is hardly significant.